The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)

Sexual Selection

Charles Darwin (1809–1882)

1871

Charles Darwin, in Origin of Species (1859), proposed that evolution was based on natural selection. Whereas the book provided a mere fleeting allusion to human evolution, hinting that it would be considered at a later day—this was sufficient to ignite passionate debate. It implied that humans had evolved from lower life forms and directly challenged the Book of Genesis. Twelve years later, in his The Descent of Man, and Selection in Relation to Sex (1871), Darwin specifically extended his theory to humans.

In the first of the two-volume, 900-page Descent, Darwin sought to provide evidence that all humans are a single species that have descended from a common apelike ancestor and had developed by evolution, as had other species; in 1871, no human fossil evidence had been discovered. He noted the similarities between humans and other primates and argued that human mental and emotional capabilities are not uniquely human traits, but that they differed in degree but not in kind from other higher animals.

Darwin then argued, from an evolutionary perspective, for the commonality and equality of the human race. He rejected the theory of polygenesis (a concept embraced by a number of prominent biologists) that the human races are derived from different lineages, created separately, with some races being inferior. Rather, he supported the theory of monogenesis, that all humans evolved from a common origin, and that the differences between the races—skin color, hair type—were all superficial; when taken in its totality, all humans closely resemble one another.

The theory underlying sexual selection was first briefly proposed by Darwin in Origins and later examined in extensive detail as it relates to humans and animals in Descent. Whereas the driving force underlying natural selection is survival, sexual selection is based on the need to reproduce. Darwin envisioned two kinds of “sexual struggles”: between members of the same sex each vying for a member of the opposite sex, and between members of the opposite sex seeking to attract them. In the latter case, the object of the attention, usually the female, selects the more desirable partner.